1. Field of the Invention
The present invention relates to rotary shredding apparatus and more particularly to a control system for operating the rotary hydraulic motor so that if the shredder jams because of the character of material thrown into the hopper it will automatically correct the jammed condition by going into a reverse rotational mode for a limited time period.
2. Description of the Prior Art
The presently known prior art relating to material shredding apparatus of rotary type driven by hydraulic fluid pressure motors or by electrical motors includes such examples as are disclosed in U.S. Patents Panning et al No. 3,502,276 of Mar. 24, 1970; Schwarz No. 3,845,907 of Nov. 5, 1974; Goldhammer No. 3,860,180 of Jan. 14, 1975; Cunningham et al No. 3,868,062 of Feb. 25, 1975; Kaczmarek No. 3,981,455 of Sept. 21, 1976; and Culbertson et al No. 4,034,918 of July 12, 1977.
It is also known that hydraulic pressure fluid systems can be operated in opposite directions by flow reversing means responsive to manual operation, or be electrically operated in response to fluid pressure means. Presently known prior art U.S. Patents in this class of systems includes Erickson No. 2,674,231 of Apr. 6, 1954; MacMillin No. 2,984,985 of May 23, 1961; Clar No. 3,336,861 of Aug. 22, 1967; and Anderson No. 3,366,016 of Jan. 30, 1968.
General information on hydraulic motors and components, and on fluid motor circuits is available in the Fluid Power Handbook & Directory 1974-75 by Hydraulics & Pneumatics.
There are problems with the electric motor and with the hydraulic motor drives for shredder apparatus as these drives are known at the present time. In the electric motor drives for low horsepower motors, frequent reversing is believed to be permissible because the motor is small enough to be easily reversed without overheating or incurring the tendency to burn out. However, as electric motor horsepower increases into the 100 H.P. range or more, the danger of burn out greatly increases, and is objectionable for that reason as the source of power for driving the counterrotating cutter shafts.
The shredder apparatus currently in use employs electric motor driven hydraulic pumps which supply the necessary power for hydraulic motors of 100 H.P. and larger. Instead of reversing the electric motors in such apparatus, control means is employed to reverse the pump if employed in a closed-loop system or to reverse a flow reversing valve means without reversing the pump, as used in an open-loop system. It has been the practice to control the flow reversing valve by either a hydraulic reversing control, or by electric solenoid means, either means being subject to fluid pressure in the system. When fluid pressure responsive means is employed, as in 3,868,062 or 4,034,918 shredder apparatus, there is a high pressure "load" which develops when the cutter elements on the counterrotating shafts jam or encounter hard to shred material.
It is known in systems relying on fluid pressure responsive electrical switches that the electrical control means associated with such switches rely upon make-or-break contactor elements, as is shown in 4,034,918 or in 3,336,861. These types of electrical components are easily responsive to vibration, and the contact surfaces become pitted from frequent opening and closing operations. In fluid pressure systems there is a need for a pressure relief valve, in addition to the flow reversing valve means, so that for correct operation it is necessary to make two adjustments, one being to respond to jam conditions, and the second being to relieve the system against over-pressure damage. Wrong adjustments can be made and if the relief valve is adjusted to open at a lower pressure than the pressure responsive switch in the reversing control system, there will be no way of knowing, short of visual inspection, whether the shredder has reversed as it is suppose to do when a jam occurs.
Furthermore, in hydraulic pressure responsive control systems, the relief pressure valve setting determines the maximum pressure available, but because the pressure responsive switch for sensing a jam is set to operate at a pressure less than the relief pressure, the maximum pressure is never available for effecting the shredding function of the apparatus.